1. Field of the Invention
The present invention relates to a photoelectric conversion element having a p-MOS structure, and to a display device that uses the photoelectric conversion element as a light receiving element and has a display function and an input function.
2. Description of the Related Art
A liquid crystal display device, which is a kind of flat-panel display device, is applied to various technical fields of OA equipment, information terminals, timepieces, TVs, etc., because of its features such as light weight, small thickness and low power consumption. In particular, a liquid crystal display device using thin-film transistors (TFT devices) has good responsivity, and is thus used as a display for various kinds of electronic devices such as mobile phones, TVs and computers.
In recent years, the range of uses of the flat-panel display device has been increasing more and more, and display devices with additional data input functions of, e.g. a touch-panel type or a pen-input type, have been developed. If direct input to the screen of the display device is enabled, the user interface can remarkably be improved. However, in the touch-panel type or pen-input type display device, it is necessary to add various components for the additional data input function, leading to a complex structure of the device and an increase in cost.
As regards the flat-panel display device such as a liquid crystal display device, a technique for reducing the manufacturing cost has been developed, wherein a driving circuit, which is conventionally an external attachment component, is formed by integrating TFTs on a major surface of a light-transmissive substrate (glass substrate). If this technique is employed to fabricate photosensor elements on a glass substrate and to provide a scanning function at the same time as the formation of the driving circuit, it is expected that the manufacturing cost of the liquid crystal display device having the input function can be reduced and the added value can be enhanced.
In Jpn. Pat. Appln. KOKAI Publication No. 2004-159273, the applicant of the present application proposed a display device wherein a light receiving sensor is fabricated in a pixel array section. Further in Jpn. Pat. Appln. KOKAI Publication No. 2004-119719, the applicant proposed a diode for a photosensor, which is usable in this kind of display device.
In the case where the light receiving sensor is fabricated in the pixel array section, as in the invention of KOKAI Publication No. 2004-159273, if the TFT for the driving circuit and the light receiving sensor can be fabricated in the same fabrication step, the manufacturing process can be simplified and the manufacturing cost can greatly be reduced.
As processes for fabricating a driving circuit on a glass substrate, there are known a C-MOS process in which the circuit is constituted by both an n-channel TFT and a p-channel TFT, and a p-MOS process in which the circuit is constituted by a p-channel TFT alone. The former process is advantageous in that a high-level driving circuit can be fabricated, but the manufacturing cost is increased. Thus, the former process is applied to the manufacture of small-sized panels, wherein many glass substrates can be obtained from a single glass substrate. The latter process is applied only to the manufacture of driving circuits with simple structure, but the manufacturing cost can be reduced. Thus, the latter process is applied mainly to large-sized panels.
In order to realize manufacture of a large-sized panel while reducing the manufacturing cost, it is necessary to fabricate a light receiving element by the p-MOS process. However, as disclosed in the invention of Jpn. Pat. Appln. KOKAI Publication No. 2004-119719, the conventional light receiving element (photosensor element) has the PIN diode structure. Thus, when the light receiving element is to be fabricated, there is no choice but to adopt the C-MOS process. As a result, there is such a problem that the manufacturing cost increases and the C-MOS process is applicable only to small-sized panels.